Induction motor drive arrangement

ABSTRACT

A self-excited induction motor variable speed drive using a known self-excitation method in which a capacitor is connected in parallel with the motor. Power is supplied by a supply convertor, a D.C. link and a motor convertor, the latter running at the motor frequency. The motor convertor includes a current bypass switching circuit comprising a capacitor bank connected to the motor terminals and to a neutral point between a pair of thyristors connected across the D.C. link.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to induction motor drive arrangements andparticularly to such arrangements employing a motor convertor suppliedfrom a controllable D.C. source.

2. Description of the Related Art

Such arrangements are known in which the D.C. source is provided by athree-phase thyristor rectifier bridge, the `supply convertor`, whichcan be phase controlled to determine the D.C. link current between thesupply convertor and the motor convertor. One or more D.C. reactors inthe D.C. link provide current inertia to maintain the motor convertorcurrent during switching operations.

In addition, it is known to provide a capacitor bank in parallel with aninduction motor to provide external excitation, since an inductionmotor, unlike a synchronous motor for example, will not generate voltageand provide its own excitation.

While an induction motor can be made to operate with excitationcapacitors in this way, there is no direct control of the speed.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an induction motordrive which offers speed control over a wide range and constant orcontrollable torque over this range.

According to the present invention an induction motor drive arrangementcomprises a controllable D.C. source, reactance means providing D.C.current inertia, a motor convertor circuit comprising a thyristor-bridgewhose input is connected to the D.C. source and whose A.C. output isconnected to motor supply terminals for connection to an inductionmotor, capacitive excitation means connected to the motor supplyterminals for maintaining induction motor excitation, and a commutationcircuit comprising commutating capacitance connected between each of themotor supply terminals and a commutation neutral point, and a bypasspath comprising two thyristor arms in series between input terminals ofthe thyristor bridge, the junction of the two thyristor arms beingconnected to the neutral point, the thyristors of said bypass arms beingfired to bypass current from said motor convertor thyristors to saidcommutating capacitance, the thyristors of said motor convertor bridgebeing fired cyclically in dependence upon the voltage across saidcommutating capacitance, and said D.C. source current being controlledin dependence upon required motor speed and torque.

The controllable D.C. source is preferably a thyristor bridge convertorhaving a phase-control firing circuit for controlling the current supplyto the motor convertor.

The capacitive excitation means preferably comprises a bank ofcapacitors connected symmetrically to the motor supply terminals.

A firing circuit for the motor convertor bridge thyristors and thebypass arm thyristors is preferably initiated by voltages betweenneutral points of the commutation and excitation capacitors.

BRIEF DESCRIPTION OF THE DRAWINGS

An induction motor drive arrangement will now be described, by way ofexample, with reference to the accompanying drawings, of which:

FIG. 1 is a diagram of an induction motor drive circuit fed from an A.C.mains;

FIG. 2 is a waveform diagram for a particular operating condition of thecircuit of FIG. 1;

FIG 3 is a motor/convertor operating characteristic for the circuit ofFIG. 1; and

FIG. 4 is an overall block diagram of the drive circuit includingvarious control features.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 of the drawings, a controllable D.C. source isprovided by a thyristor bridge supply convertor 11 connected to a3-phase A.C. supply 13. The supply convertor is connected to a motorconvertor 15 which also comprises a 3-phase thyristor bridge. The twoconvertors are connected by a D.C. link 17, in one or both of which aD.C. link reactor 19 is connected. The inertial effect of this reactortends to stabilise the current and maintain conduction in the motorconvertor during commutation. Control of the current level in the D.C.link is largely effected by the supply convertor, as will be explained.

The 3-phase output of the motor convertor, operating as an inverter, isapplied to an induction motor 21 by way of motor terminals A, B and C.Connected across the motor terminals is a bank of capacitors 23providing external excitation for the motor enabling it to generateterminal voltage necessary for commutation of the motor convertor. Thisis achieved by the capacitor bank providing a low loss path for themotor currents which could not otherwise be maintained.

In an arrangement as so far described, there is no control of the motorspeed. Variable frequency drives for induction motors have been proposedbut they have suffered from disadvantages either in complexity or in aneed for high speed turn-off thyristors which are expensive. The presentinvention provides an extremely simple commutating circuit and providesthe additional important advantage that low-speed, and thereforerelatively cheap, thyristors can be used for the motor convertor withlittle or no fear of switching problems.

A bypass path consisting of two thyristors T7 and T8 in series, isconnected across the input terminals 14 and 16 of the motor convertorproviding, in effect, two further arms to the bridge, poled similarly tothe existing arms. A bank 22 of three capacitors C_(A), C_(B) and C_(C)are star connected, one to each output line of the motor convertor, theother capacitor terminals being connected to a neutral point N (the`commutation neutral`). Inductors L_(A), L_(B) and L_(C) are connectedin series in the output of the thyristor bridge. These inductors arepurely to suppress rapid transients and are not essential to theoperation.

The neutral point N is connected to the junction 18 of the two bypassthyristor arms.

The thyristors of the motor convertor 15 are referenced T1 to T6 in theorder of their firing. A firing circuit 25 is required to respond to thevoltage across the commutation capacitors 22 and is conveniently drivenby the voltage between the neutral points M and N and the voltages atmotor terminals A, B and C.

Finally, the bypass thyristors T7 and T8 are fired alternately one aftereach firing of the thyristors T1 to T6, as will be explained withreference to FIG. 2, and in dependence upon the requirements of themotor characteristic of FIG. 3.

Referring now to FIG. 2, this shows current and voltage waveformsassociated with the motor and motor convertor. The three-phase sinewaves represent the output voltages of the motor convertor, i.e. at themotor terminals A, B and C.

The bypass arms T7 and T8 are fired alternately, T7 while anodd-numbered bridge thyristor is conducting and T8 while aneven-numbered thyristor is conducting. The effect of firing T7 or T8 isto deprive the associated conducting bridge thyristor of current andbypass the current to or from the commutating capacitor bank 22. Thebridge thyristor is thus cut-off and firing of the next bridge thyristorof the same pole can be effected. Thus, it may be seen from FIG. 2 that,when T8 is fired while T6 is conducting, the latter turns off and allowsT2 to be fired. T8 therefore allows the circuit current to betransferred from T6 to T2, the current being removed from T6 for aperiod of time sufficient for T6 to regain its blocking capability andfor the commutating capacitors 22 to recharge. Once T2 is fired thecircuit current transfers from T8 into T2.

The timing of the firing of bypass thyristors T7 and T8 is related tothe "inversion crossover" point I, being set an an angle β within ±90°of this point. The condition shown is 30° in advance and the effect ofdifferent angles will be explained with reference to FIG. 3. Thisangle βmay be seen to be 180°-α where α is the conventional delay angle fromthe `natural` conduction point at which a diode would conduct.

The firing of T8 takes the commutation neutral N up to the D.C. linkpotential at bridge input 14 while the firing of T7 takes the neutralpotential down to D.C. link potential 16. The neutral potential is shownby the trapezoidal waveform N' and the duration of the sloping portionsis determined by: the value of the commutation capacitance which ischarged or discharged by the bypass current, and the magnitude of theD.C. link current which constitutes the charging/discharging current.The bypass thyristor is cut off by the firing of the next bridgethyristor (e.g. T4 after T2), in response to the commutation capacitorvoltage reaching a predetermined level, at which point the neturalpotential remains constant at the upper or lower peak level of waveformN' until the alternate bypass thyristor is fired.

The conduction angle of each bypass thyristor is referred to as thegamma angle (γ) and is not normally allowed to exceed 30°. In theexample shown in FIG. 2 γ is 15° and β is 30°.

As β is varied in FIG. 2, all of the waveforms shift relative to thesine waves of the convertor output.

Referring now to FIG. 3, this shows the motor and motor convertorcharacteristics under control of the commutating circuit of FIG. 1. Thevertical axis is the motor current component in-phase with the motorvoltage and thus proportional to the motor torque. The horizontal axisgives, to the right, the reactive current component providing the motormagnetisation, and to the left, the excitation capacitor current.

The vertical curve CAD is the locus of the motor current vector, e.g. OAor OD, at low speed. Thus vector OC corresponds to zero speed and zerotorque while vector OD corresponds to zero speed and full torque.

Similarly, the curve AB is the locus of the motor current vector forfull speed conditions. Thus OA is the full-speed low-load current vectorand OB is the full-speed full-load current vector. The area CADBAC thusrepresents the locus of all required motor current vectors. It is thesecurrent vectors which must be provided by the motor convertor 15, takingaccount of the current drawn by the excitation capacitor bank 23.

Thus the convertor current is required to equal the vector sum of themotor current and the excitation capacitor current at all speeds withinthe control range. For example, a vector OR within the area CADBAC,representing a significant torque level and a speed approaching fullspeed (i.e. R is closer to the characteristic AB than to CD), isobtained as the difference between an invertor current vector OS and theexcitation capacitor current vector I_(c).

The full speed convertor current vectors lie within the range of OP atlow torque and OQ at full torque.

The effect of varying β in FIG. 2 can be seen in FIG. 3, β being theangle between the vertical `in-phase` convertor voltage axis and theconvertor current vector (OP in the example illustrated). Thus the phaseof the convertor current is controllable by control of β, which can bevaried on either side of the voltage axis by 90°. The magnitude of themotor convertor current (and thus of course the motor current) iscontrollable by phase control of the supply convertor 11. The motorconvertor current vector is therefore fully controllable to provide, incombination with the excitation capacitor current (which is not directlycontrollable but depends upon the motor terminal voltage and frequency),the desired motor current vector.

By control of β therefore, a torque characteristic approximatelyconstant over a large speed range can be obtained, e.g. the curve QDcovering the range full-speed to zero-speed.

OP and OQ are the low-torque and full-torque convertor current vectorsat full speed. Thus PQ is the convertor current locus for varying torqueat full speed. The similar broken curves labelled "90% speed", "75%speed", etc. are corresponding convertor current loci for torquevariation at the indicated speeds.

Curve CQ is the locus of convertor current which gives a torque varyingwith the square of the speed.

Referring now to FIG. 4, this shows a schematic diagram of the overalldrive system including those essential components shown in FIG. 1.

The induction motor 21 has terminals A,B and C to which are connectedthe excitation capacitor bank 23. Supply convertor 11, D.C. link reactor19, D.C. link 17 and motor convertor 15 connect the motor 21 to the A.C.supply.

The firing circuit 25 of FIG. 1 is shown to incorporate a number ofcomponents, some conventional and some specific to this application.Thus, timing for the bridge thyristors T1-T6 of FIG. 1 is obtained, asexplained above, by detection of a predetermined voltage magnitudeacross the commutation capacitors 22. A voltage detection circuit 27detects this level and determines the phase of the bridge-thyristorfiring pulses accordingly. The duration of the transition between thecapacitor 22 detection levels is also determined by the circuit 27 toprovide a value for γ, the bypass thyristor conduction angle. This valueis applied to a process box 29.

The frequency of the firing pulses applied by a pulse amplifier circuit31 is determined by a phase lock loop oscillator 33 in known manner independence upon the integration of a signal which departs from a pre-setvalue in the presence of an error. The oscillator output pulses, whichare basically at six times the motor frequency, i.e. at 60° electrical,control the firing of the bypass thyristors T7 and T8. With a delayangle of γ provided by the voltage detector 27 to a delay circuit 35,the oscillator output pulses also trigger the bridge thyristors T1-T6,the two sets of pulses being distributed appropriately by the pulseamplifier circuit 31.

The phase lock loop oscillator 33 responds to the motor terminal voltageby way of a transformer 37 and also to a V/f signal from a process box39. The latter signal is the ratio of the motor terminal voltage to thefrequency and is proportional to the motor flux. This signal is comparedwith a reference value imposed by a control 41 in an amplifier 43. Anychange in the motor flux produces an error which gradually increases ordecreases (as necessary) the motor convertor firing angle β and thus theamount of magnetising current and flux in the motor. The ratio signaland consequently the motor flux are thus corrected and maintainedstable.

The supply convertor 11 is controlled by a current feedback loop with aspeed control superimposed. The supply bridge thyristors are fired byway of a current amplifier 47, filter 49, firing circuit 51 and pulseamplifiers 53 in known manner. One input to the amplifier 47 is afeedback input from a transformer 55 detecting supply current magnitude.

A second input to the amplifier 47 is derived from a speed referencecontrol 57 which is set at the desired speed. A ramp circuit 59 convertsstep changes to gradual changes and the resulting signal is applied to aspeed amplifier 61 for comparison with a speed signal fed back from theoscillator 33, the voltage/frequency signal also constituting a speedsignal. The difference signal output from amplifier 61 is applied to afrequency amplifier 63 which also receives the reference speed andfeedback speed signals. The resulting output is applied to the currentamplifier 47 so providing control of the D.C. link current and hence thecurrent flowing in the motor convertor and motor.

The basic principles are that the control provided for the supplyconvertor controls the D.C. line current and the torque in the motor inresponse to the speed requirement and that the control over the motorconvertor alters the angle β of the convertor current so as to maintainthe motor excitation and flux. To achieve this effectively additionalsignals are included between these two systems. The current demandsignal from a current limit circuit 65 is used to assist in stabilisingthe flux control under all conditions and the signal from Box 29 to Box69 provided is used to modify the effectiveness of the current controlin dependence on the angle β.

I claim:
 1. An induction motor drive arrangement comprising acontrollable D.C. source, reactance means providing D.C. currentinertia, a motor convertor circuit comprising a thyristor bridge whoseinput is connected to said D.C. source and whose A.C. output isconnected to motor supply terminals for connection to an inductionmotor, capacitive excitation means connected to said motor supplyterminals for maintaining induction motor excitation, and a commutationcircuit comprising commutating capacitance connected between each ofsaid motor supply terminals and a commutation neutral point, and abypass path comprising two thyristor arms in series between inputterminals of said thyristor bridge, the junction of said two thyristorarms being connected to said neutral point, the thyristors of saidbypass arms being fired to bypass current from said motor convertorthyristors to said commutating capacitance, the thyristor of said motorconvertor bridge being fired cyclically in dependence upon the voltageacross said commutating capacitance, and said D.C. source current beingcontrolled in dependence upon required motor speed and torque.
 2. Aninduction motor drive arrangement according to claim 1, wherein saidcontrollable D.C. source is a thyristor bridge convertor having aphase-control firing circuit for controlling the current supply to themotor convertor.
 3. An induction motor drive arrangement comprising acontrollable D.C. source, reactance means providing D.C. currentinertia, a motor converter circuit comprising a thyristor bridge whoseinput is connected to said D.C. source and whose A.C. output isconnected to motor supply terminals for connection to an inductionmotor, capacitive excitation means connected to said motor supplyterminals for maintaining induction motor excitation, and a commutationcircuit comprising commutating capacitance connected between each ofsaid motor supply terminals and a commutation neutral point, and abypass path comprising two thyristor arms in series between inputterminals of said thyristor bridge, the junction of said two thyristorarms being connected to said neutral point, the thyristors of saidbypass arms being fired to bypass current from said motor convertorthyristors to said commutating capacitance, the thyristors of said motorconvertor bridge being fired cyclically in dependence upon the voltageacross said commutating capacitance, said D.C. source current beingcontrolled in dependence upon required motor speed and torque, and saidcapacitive excitation means comprising a bank of capacitors connectedsymmetrically to said motor supply terminals.
 4. An induction motordrive arrangement comprising a controllable D.C. source, reactance meansproviding D.C. current inertia, a motor convertor circuit comprising athyristor bridge whose input is connected to said D.C. source and whoseA.C. output is connected to motor supply terminals for connection to aninduction motor, capacitive excitation means connected to said motorsupply terminals for maintaining induction motor excitation, and acommutation circuit comprising commutating capacitance connected betweeneach of said motor supply terminals and a commutation neutral point, anda bypass path comprising two thyristor arms in series between inputterminals of said thyristor bridge, the junction of said two thyristorarms being connected to said neutral point, the thyristors of saidbypass arms being fired to bypass current from said motor convertorthyristors to said commutating capacitance, the thyristors of said motorconvertor bridge being fired cyclically in dependence upon the voltageacross said commutating capacitance, said D.C. source current beingcontrolled in dependence upon required motor speed and torque, and saidcommutating capacitance comprising a star-connected capacitor bank. 5.An induction motor drive arrangement comprising a controllable D.C.source, reactance means providing D.C. current inertia, a motorconvertor circuit comprising a thyristor bridge whose input is connectedto said D.C. source and whose A.C. output is connected to motor supplyterminals for connection to an induction motor, capacitive excitationmeans connected to said motor supply terminals for maintaining inductionmotor excitation, and a commutation circuit comprising commutatingcapacitance connected between each of said motor supply terminals and acommutation neutral point, and a bypass path comprising two thyristorarms in series between input terminals of said thyristor bridge, thejunction of said two thyristor arms being connected to said neutralpoint, the thyristors of said bypass arms being fired to bypass currentfrom said motor convertor thyristors to said commutating capacitance,the thyristors of said motor convertor bridge being fired cyclically independence upon the voltage across said commutating capacitance, saidD.C. source current being controlled in dependence upon required motorspeed and torque, and a firing circuit for said motor convertor bridgethyristors connected between said commutation and excitation neutralpoints.
 6. An induction motor drive arrangement comprising acontrollable D.C. source, reactance means providing D.C. currentinertia, a motor convertor circuit comprising a thyristor bridge whoseinput is connected to said D.C. source and whose A.C. output isconnected to motor supply terminals for connection to an inductionmotor, capacitive excitation means connected to said motor supplyterminals for maintaining induction motor excitation, and a commutationcircuit comprising commutating capacitance connected between each ofsaid motor supply terminals and a commutation neutral point, and abypass path comprising two thyristor arms in series between inputterminals of said thyristor bridge, the junction of said two thyristorarms being connected to said neutral point, the thyristors of saidbypass arms being fired to bypass current from said motor convertorthyristors to said commutating capacitance, the thyristors of said motorconvertor bridge being fired cyclically in dependence upon the voltageacross said commutating capacitance, said D.C. source current beingcontrolled in dependence upon required motor speed and torque, and meansfor inhibiting the firing of thyristors in said bypass path above apredetermined operating frequency thereby permitting load commutation ofsaid motor convertor above said predetermined frequency.
 7. An inductionmotor drive arrangement comprising a controllable D.C. source, reactancemeans providing D.C. current inertia, a motor convertor circuitcomprising a thyristor bridge whose input is connected to said D.C.source and whose A.C. output is connected to motor supply terminals forconnection to an induction motor, capacitive excitation means connectedto said motor supply terminals for maintaining induction motorexcitation, and a commutation circuit comprising commutating capacitanceconnected between each of said motor supply terminals and a commutationneutral point, and a bypass path comprising two thyristor arms in seriesbetween input terminals of said thyristor bridge, the junction of saidtwo thyristor arms being connected to said neutral point, the thyristorsof said bypass arms being fired to bypass current from said motorconvertor thyristors to said commutating capacitance, the thyristors ofsaid motor convertor bridge being fired cyclically in dependence uponthe voltage across said commutating capacitance, said D.C. sourcecurrent being controlled in dependence upon required motor speed andtorque, and inductors coupled to said capacitive excitation means toform a harmonic filler.